She doesn't appear to have a heavy worm load nor blackhead, I believe you are dealing with a laying issue, Her abdomen looks swollen..impacted oviduct or internal layer maybe?
My personal opinion is that it is not cocci. (by the way, medicated feed does NOT prevent cocci, no matter what they say).
Blackhead is not very common in chickens, at least they don't seem to suffer from it like turkeys do, so I'd more veer toward what Pine Grove said, though her age is quite young. I just lost a 16 month old hen to a massive ovarian infection, something I'd only seen once in a hen of that age. Anything is possible at any age if she's laying, though. The yellow poop could be egg yolk. Even with worms, I've never seen bright yellow poop. I do see it when loose yolk is expelled. I agree with casportpony that you may need to treat for ecoli-that won't be penicillin, but something like Baytil or amoxicillin. It may already be too late, but time is of the essence now.
sammy56 you need to start your own thread so your question isn't lost in this one.